Anechoic and hemi-anechoic chambers are designed to mimic an acoustic free field inside of an enclosed environment. An acoustic free field is a region where there are no acoustically reflective surfaces or effects from such surfaces.
Current and historical anechoic chambers have all been constructed in similar methods. FIG. 1 is an acoustic chamber 10 having a solid outer wall 12 constructed to isolate the acoustic environment 14 inside the chamber from that outside the chamber. The outer wall 12 is typically constructed using modular steel panel systems or by conventional techniques such as concrete, cinder blocks, or studs and drywall. The interior of the outer wall 12 is then lined with an acoustic absorber system 16. The absorber system 16 absorbs nearly all of the acoustic energy within its frequency range of operation creating a free field. This absorber system 16 is typically a wedge or a tuned wedge/wall system where the wedges operate in conjunction with the wall panels. In a tuned wedge/wall system, the outer wall 12 of the chamber 10 is solid steel and the inner wall 20 is usually perforated steel. The inside of the wall enclosure 18 is filled with a variety of different materials depending on what the designer is trying to achieve.
In a tuned wedge/wall system, where the inner wall is perforated steel, the outer wall 12 of the acoustic chamber 10 in this case is solid and can be steel or one of the other materials mentioned above. The solid outer wall 12 is reflective and impacts the free field performance of the chamber below the absorber cutoff frequency. At low acoustic frequencies, the acoustic energy is not totally absorbed by the absorber system 16 and some acoustic energy passes through the perforated inner wall 20. Some of the acoustic energy passing through the inner wall 20 is absorbed by material of the wall enclosure 18. However, some of the acoustic energy will reflect from the inner surface of the solid outer wall 12 and will propagate back toward the interior 14 of the acoustic chamber 10. This is undesirable because it degrades the low frequency performance of the acoustic chamber 10.
In order to improve the low frequency performance of an anechoic chamber, conventional solutions call for the chamber size to increase and the depth of the absorber system 16 to increase. This greatly increases cost and size of these chambers and limits the ability of users to obtain a chamber that meets their low frequency requirements.